No production project can use Haml until it is supported by Firefox, Chrome, Safari and Internet Explorer. It used to be that "enterprise" applications could be IE only, but I think the days of that are over. Standards aren't imposed from on-high, they are agreed to by browser vendors.
So really to have Haml made a standard, you'd need to get Apple, Google, the Mozilla Foundation and Microsoft to agree. This isn't trivial. These companies are going to generally concentrate on expanding capabilities rather than cleaning up existing features.
Haml looks nice to work with, but it won't actually improve download sides as all modern browsers and servers support compression. Compressed Haml and Html are likely to be about the same size. (Plus, most of the download time for the average website is in downloading images and script code.)
Now keep in mind that few people write in Html anymore. People use frameworks that spit out Html as an end product. Not only would this hurt the adoption of Haml directly, as none of these frameworks will support it, but it obviates the need for it, because the underlying markup language is only seen by the computer.
From the browser vendor point of view, they can improve an existing feature slightly (by supporting something like Haml, which gives cleaner pages) or they can add something entirely new, like WebGL. The latter just has more bang for the buck.